This invention relates to positive fluid displacement devices and more particularly to devices of the piston type for precision fluid delivery.
U.S. Pat. No. 6,162,030 describes a Positive Fluid Displacement Device (PFDD) which is the basis of the current invention. The object of the current invention is to improve the design of the patented device. The improved design described herein provides better performance, includes a broader range of applications, improves manufacturability, broadens tolerances, eliminates parts, eases assembly and lowers cost. However, the principles of operation of the PFDD are unchanged and since those principles are fully described in FIGS. 1A-1D of the referenced patent, they are not repeated herein.
The design has been improved by replacing separate metal parts with single parts using metal or plastic material. The coupling of some components has been modified to allow significantly greater variation in tolerances without reduction in accuracy of fluid delivery and performance of the PFDD. Pliable members are used to position parts with respect to each other for quieter operation, easier assembly and broadening of the tolerances. The configuration of the seals has been modified to eliminate metal parts and to allow the use of different sealing materials in order to meet chemical compatibility requirements with a minimum of changes.
The use of glass and ceramic material as wetted parts in the device requires careful mounting since those parts cannot be made to the same degree of accuracy as can plastic and metal parts. Therefore, a design which allows significant tolerance in the dimensions of the wetted parts eliminates secondary machining or grinding, thus producing a lower cost device.
Design improvements in the manifold permit variation in the internal configuration of the manifold passageways to meet different customer requirements, without change in the basic PFDD configuration. Improved mounts for motor connection permit different types of motors to be used, and provides improved rigidity in a minimum amount of space. The inclusion of an optional gearbox permits the use of a smaller motor by increasing the torque available from the motor.
One aspect of this invention involves the replacement of the multi-part four-piston assembly of the Fluid Displacement Module (FDM) described in the referenced patent with two single parts, each acting as a double-headed piston. Each part is such that it can nest into another identical part, thus providing four pistons in the same plane but oriented approximately 90xc2x0 apart. The two double-headed pistons are rotatably connected together in a plane perpendicular to the axis of the crankshaft. They are mounted concentrically around the crankpin, so the 90xc2x0 separation of the pistons is not established by the pistons, but rather by the position of two cylinder carriages. The position of the carriages is defined by grooves in the housing of the PFDD.
Each piston head also acts as a piston seal and each seal is secured directly to the end of the piston. The double-headed piston slides through the carriage for ease of assembly. Like the patented device, each one has a protrusion to fit inside the port in the cylinder head to reduce dead volume.
A second aspect of this invention involves a cushioned support for holding the port plate that floats along an axis perpendicular to the axis of the crankshaft. The port plate is captivated to the housing by pliable members such as elastomeric cords which are embedded into the housing. This allows micromotion of the port plate inside the housing, without any part of the port plate directly in contact with the housing. This eliminates rubbing of the port plate directly against the housing, and provides for wide tolerance in the machining of the housing and the port plate. It also provides a spring action on the port plate against the manifold, thus insuring good sealing contact on seals located between the manifold and port plate without preventing the port plate from floating against the cylinder head.
A third aspect of this invention also relates to cushioning the cylinder heads as they act against the manifold. The cylinder heads are slidably mounted on plastic rails that are also slidably mounted into grooves machined into the housing of the PFDD. Behind the rails, embedded inside the bottom of the grooves, is a pliable buffering member which acts as a spring pushing the cylinder heads against the manifold. The intimate and continuous contact of the cylinder heads against the manifold provides a silent operation without the need to machine the depth of the grooves and the width of the cylinder heads to high precision.
A fourth aspect of this invention is to provide controlled pressure on the port plate toward the cylinder head in order to maintain zero leakage. This is accomplished by providing a resilient urging member between the housing and the port plate to urge the port plate against the cylinder head. The urging member, may be an elastomeric material or a spring. If a spring is used, the port plates are provided with a groove on the surface opposite the surface sliding against the cylinder head. The groove captivates a metal spring that applies pressure to the center of the port plate. The length and thickness of the spring precisely controls its force against the port plate. The two opposite ends of the spring react against the internal surface of the housing. This design reduces clearance between the top of the port plate and the external surface of the housing to near zero, thus reducing overall dimensions of the housing.
A fifth aspect of this invention is to provide a cushioned mounting for essentially brittle ceramic or glass cylinders which are loosely mounted inside the cylinder head and the carriage. At the cylinder head, a compliant sealing member provides a seal between cylinder and the cylinder head that acts in a direction parallel to the sliding surface of the cylinder head, thereby avoiding pressure on the cylinder head in a direction perpendicular to the sliding surface. In that manner, distortion of the flatness of the sliding surface of the cylinder head is prevented since there is no contact pressure between the cylinder and the cylinder head, except through the sealing member. The sealing member, which may be an O-ring, also acts to center the cylinder inside the counterbore of the cylinder head. At the other end of the cylinder, a compliant washer, made of Teflon for example, is interposed between the cylinder and the carriage to prevent direct contact between the cylinder and the carriage, thereby avoiding stressing the glass or ceramic cylinder when the cylinder head is assembled to the carriage.
Additionally, the area of the end surfaces of the carriage in contact with the cylinder head are reduced by providing recesses. The reduction of the contact surface area allows them to be machined and lapped to a flatness of better than two light bands.
A sixth aspect of this invention is to provide a double-layer manifold that is fastened against the PFDD housing. A first layer of a two-layer manifold has a surface, opposite to the surface in contact with the housing, with fluid passageways grooved therein. The second layer of the two-layer manifold is pressed against the first layer and seals all the grooved passageways. Connection to the fluid supply and to devices using the PFDD is done through inlet and outlet ports on the second layer. The advantage of this design is the elimination of drilling long holes in the manifold and the use of smaller cross section passageways than can be done with a long hole design. The tightness of the fluid passageways is insured between the surfaces of the manifolds by lapping them to a flatness of better than two light bands.
A seventh aspect of this invention is to directly mount the motor to the back of the PFDD, without couplings, and to have, as an option, a torque-increasing gearbox interposed between the motor and the PFDD.
The above mentioned and other features and objects of this invention and the manner of obtaining them will become more apparent, and the invention itself will best be understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawing, a description of which follows.